Permian survivors The most catastrophic mass extinction in the history of Earth had little effect on the lowliest of beasts living in the bottom-dwelling muck on the sea floor, according to a new study.

This could explain the puzzling fact that even after the worst extinction event of all time, very few new groups of benthic (bottom-dwelling) animals — like snails and clams — sprung from the wreckage.

Despite the fact that 96 per cent of marine species perished in the end-Permian extinctions, some 252 million years ago, only one of the 25 benthic lifestyles that then existed actually disappeared when life struggled on into what's called the Triassic period.

That made it harder for new groups of benthic animals to evolve in the aftermath, says William Foster and Richard Twitchett of Plymouth University in the UK.

"Globally, the Early Triassic benthic ecosystem functioned much like a ship manned by a skeleton crew. Each post was occupied, but by only a few individual taxa (groups)," says Foster.

That skeleton crew appeared to be enough, however, to keep intact the lifestyles that have defined benthic animals ever since.

To come to that conclusion, Foster and Twichett had the daunting task of sorting through the worldwide benthic fossil record from that pivotal time in Earth's history.

Previous studies have looked at the groups that went extinct, of course, as well as how new lifestyles "went viral" and spread through the world. But this is the first to look at the greatest extinction solely terms of animal roles — or the 'ecospace' — that animals filled.

"We sort of ignored (the animals') names for this study," explains Foster. One advantage of this approach, he says, is that when the fossil record lacks evidence of a particular animal for a period of time, you can sometimes follow the lifestyle to fill in the blanks.

"The ecospace occupied by a marine organism is characterized by three ecological variables: mobility, feeding mechanism and living location," explains Martin Aberhan of the Museum für Naturkunde, Leibniz Institute for Research on Evolution and Biodiversity in Germany.

None of this means to suggest that the extinction event, which was accompanied by a severe bout of global warming, went entirely unnoticed by sea-floor inhabitants.

"The number of lifestyles didn't change, but that doesn't mean nothing happened," says Foster. In fact there was a turnover in the signature bottom dwellers.

In the late Permian the seas were dominated by crinoids and brachiopods. These signature species, which can be found fossilised around the world, were almost wiped out. That set the stage for the early ancestors of today's burrowing clams and snails to take over.